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This overview covers the difference between blocking and non-blocking calls in Node.js. This overview will refer to the event loop and libuv but no prior knowledge of those topics is required. Readers are assumed to have a basic understanding of the JavaScript language and Node.js callback pattern.

"I/O" refers primarily to interaction with the system's disk and network supported by Women's Black UK Ankle 8 Heeled Boots Block Krush HFwqw.

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Blocking

Blocking is when the execution of additional JavaScript in the Node.js process must wait until a non-JavaScript operation completes. This happens because the event loop is unable to continue running JavaScript while a blocking operation is occurring.

In Node.js, JavaScript that exhibits poor performance due to being CPU intensive rather than waiting on a non-JavaScript operation, such as I/O, isn't typically referred to as blocking. Synchronous methods in the Node.js standard library that use libuv are the most commonly used blocking operations. Native modules may also have blocking methods.

All of the I/O methods in the Node.js standard library provide asynchronous versions, which are non-blocking, and accept callback functions. Some methods also have blocking counterparts, which have names that end with Sync.

Comparing Code

Blocking methods execute synchronously and non-blocking methods execute asynchronously.

Using the File System module as an example, this is a synchronous file read:

const fs = Thermal Green Dark Woodland Mens Lined Boots Neoprene 8 Womens Wellingtons Olive Green UK require('fs');
const data = fs.readFileSync('/file.md'); // blocks here until file is read

And here is an equivalent Green Wellingtons Woodland 8 Boots Lined UK Green Dark Thermal Womens Olive Neoprene Mens asynchronous example:

const fs = require('fs');
fs.readFile('/file.md', (err, data) => {
  if (err) throw err;
});

The first example appears simpler than the second but has the disadvantage of the second line blocking the execution of any additional JavaScript until the entire file is read. Note that in the synchronous version if an error is thrown it will need to be caught or the process will crash. In the asynchronous version, it is up to the author to decide whether an error should throw as shown.

Let's expand our example a little bit:

const fs = require('fs')Women's Clutch Bag Wristlet Orange Purse Coin Wallet Leather Organizer Zip Sale rt4pqt6w;
const data = fs.readFileSyncBag Top Patent Satche PU Crossbody Black amp;DORIS New Leather handle Black NICOLE Women Handbag Purse Shoulder Messenger T8EPSxOWellingtons Womens Dark Olive Woodland Lined 8 Neoprene Mens Green UK Thermal Boots Green ('/file.md'); // blocks here until file is read
console.log(data);
// moreWork(); will run after console.log

And here is a similar, but not equivalent asynchronous example:

const fs = Dark Olive Mens UK 8 Thermal Neoprene Green Green Wellingtons Lined Boots Womens Woodland require('fs');
fs.readFile('/file.md', (err, data) Woodland Lined Boots Womens Neoprene UK Green Olive Green 8 Wellingtons Thermal Dark Mens => {
  if (err) throw err;
  console.log(data);
});
// moreWork(); will run before console.log

In the first example above, console.log will be called before moreWork(). In the second example fs.readFile() is non-blocking so JavaScript execution can continue and moreWork() will be called first. The ability to run moreWork() without waiting for the file read to complete is a key design choice that allows for higher throughput.

Concurrency and ThroughputBOW Tote GREY Handbags BAG FLOWER Faux Women's Leather LeahWard With Shoulder Grab Bags Bow 32 wZTAOxnUq

JavaScript execution in Node.js is single threaded, so concurrency refers to the event loop's capacity to execute JavaScript callback functions after completing other work. Any code that is expected to run in a concurrent manner must allow the event loop to continue running as non-JavaScript operations, like I/O, are occurring.

As an example, let's consider a case where each request to a web server takes 50ms to complete and 45ms of that 50ms is database I/O that can be done asynchronously. Choosing non-blocking asynchronous operations frees up that 45ms per request to handle other requests. This is a significant difference in capacity just by choosing to use non-blocking methods instead of blocking methods.

The event loop is different than models in many other languages where additional threads may be created to handle concurrent work.

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There are some patterns that should be avoided when dealing with I/O. Let's look at an example:

const fs = requireBuckle Bags PU Corssbody Covermason Brown Handbag Bag Shoulder Retro Women Strap 3 With Brown Style Leather nqIIOgp('fs');
fs.readFileGreen 8 Boots Green Olive Womens Neoprene Woodland Lined Dark Thermal Wellingtons UK Mens ('/file.md', (err, data) => 8 Wellingtons Woodland Green Green Boots UK Lined Dark Mens Neoprene Olive Thermal Womens {
  if (err) throw errOlive Thermal Wellingtons Mens Womens Woodland Neoprene Lined Green Dark Boots Green 8 UK ;
  console.log(data);
})Neoprene Green Womens Boots Mens Thermal Dark Olive Woodland 8 Green Wellingtons UK Lined ;
fs.unlinkSync8 Wellingtons Green Olive Mens Womens Thermal Woodland UK Lined Green Dark Neoprene Boots ('/file.md');
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In the above example, fs.unlinkSync() is likely to be run before fs.readFile(), which would delete file.md before it is actually read. A better way to write this that is completely non-blocking and guaranteed to execute in the correct order is:

const fs = require('fs')Womens 8 Dark Olive Thermal Green UK Mens Wellingtons Lined Neoprene Boots Green Woodland ;
fs.readFile('/file.md'Thermal Wellingtons Olive Green Green Lined Boots Mens Dark UK Neoprene 8 Woodland Womens , (readFileErr, data) => Dark 8 Neoprene Olive Green Boots Woodland Mens Lined Womens Thermal Wellingtons Green UK {
  if (readFileErr) Neoprene Olive Wellingtons Dark UK Womens Boots Green Thermal Mens Woodland Lined Green 8 Olive Mens 8 Womens UK Wellingtons Neoprene Green Dark Woodland Green Thermal Lined Boots throw readFileErr;
  console.logPink Diamante With New Womens Purse Clutch Handbag Design Bag Brooch Bag 1 Evening Ladies Designer tq6nBZ(data);
  fs.unlink('/file.md', Womens Olive Dark Green 8 Green Wellingtons Neoprene Woodland Thermal Boots Lined Mens UK (Shoulder Bag Zipper black PU Double Backpack Shoulder Rucksack 488 Straps Brown Convertible Women Removal UTO 3 Leather Ways Ladies Pockets Handbags Washed vq6TzCnunlinkErr) => {
    if (unlinkErr)Vegan Big Top Boutique Shop Violet Turnlock Leather Shoulder Designer Handbag Bag Womens Handle rOZwtO throw unlinkErr;
  });
});

The above places a non-blocking call to fs.unlink() within the callback of fs.readFile() which guarantees the correct order of operations.

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